The present invention relates to carbon fibre paper which is used as the current collector of a polymer electrolyte fuel cell.
In addition to a current collecting function the current collectors of a polymer electrolyte fuel cell need to allow diffusion/permeation of the substances which participate in the electrode reactions. Furthermore, the material from which the current collectors are composed needs to possess electroconductivity and the strength to withstand gas diffusion/permeation and handling, and also the strength to withstand compression at the time of electrode production and during cell assembly, etc. When compared to the characteristics demanded of tile current collectors for a phosphoric acid fuel cell, in the case of the current collector of a polymer electrolyte fuel cell the strength of the polymer electrolyte membrane is high so the strength of the current collectors need only be enough to withstand handling, Furthermore, the corrosion resistance need only be low, so the polymer material selection range is broad. However on the other hand, since the strength and resistance of the polymer electrolyte membrane is high, its thickness is reduced, so it is necessary that there be no projecting regions in the current collectors which could cause a short circuit through the polymer electrolyte membrane. Additionally, since the polymer electrolyte membrane the catalyst layers and current collectors are usually coupled together by application of pressure, it is necessary that the current collectors be undamaged, not merely by the applied pressure at the time of cell assembly but also by the pressure at the time of the integral coupling, and it is necessary that a short circuit does not occur through the polymer electrode membrane.
As the material used for the current collectors of such a polymer electrolyte fuel cell, there is known a porous carbon sheet formed by binding together short carbon fibre with carbon, as described in Japanese Unexamined Patent Publication (Kokai) Nos 6-20710, 7-326362 and 7-220735. However, since such porous carbon sheet is produced by firstly preparing an aggregate of short fibre comprising carbon fibre or carbon fibre precursor fibre, then impregnating or mixing with resin and firing, the production costs are high. Again, in the case where the density is low, there is also the problem that the binding carbon is readily damaged by the pressure applied at the time of electrode production or cell assembly.
The use of a paper-form short carbon fibre aggregate as a current collector is proposed in Japanese Unexamined Patent Publication (Kokai) No. 7-105957 as a method for resolving the problem of production costs. With such a current collector since binding with carbon is not carried out, it is necessary to apply a thickness direction pressure not just at the time of the integral coupling of the polymer electrolyte, catalyst layers and current collectors, but also when used as a cell in order to lower the electrical resistance in the thickness direction. However, in these inventions, no consideration is given to the lowering of the resistance or to preventing damage to the current collector at the time of pressure application. Moreover, since the short carbon fibres are randomly arranged when pressure is applied at the time of the electrode production or cell assembly, short carbon fibres facing in the thickness direction readily pierce the polymer electrolyte membrane, bringing about a short circuit with the facing electrode, and breakage of the short carbon fibre also readily occurs. Again, in Japanese Unexamined Patent Publication (Kokai) No. 8-7897, there is described the adhesion of the short carbon fibre in a state of entanglement with carbon particles contained in the diffusion layer at the surface on the diffusion layer side of a coupled body comprising the electrolyte membrane forming the electrodes, the catalyst reaction layers and diffusion layers, but since the short carbon fibre is fixed by entanglement with carbon particles in the diffusion layer, the short carbon fibres emerging at the surface are all at an angle in the terms of the plane of the coupled body, and at the time of pressure application when assembling the cell the short carbon fibres readily pierce the polymer electrolyte membrane bringing about a short circuit with the facing electrode, and breakage of the short carbon fibre also readily occurs. Moreover, since the layer of short carbon fibre is thin, there is low gas diffusion/permeation in the planar direction of the layer, and it is necessary to provide a diffusion layer separately.
The present invention has been made in view of the aforementioned problems of the prior art and has as its objective to provide a carbon fibre paper for use as the current collectors in polymer electrolyte fuel cells where there is little concern about a short circuit with the facing electrode occurring, where there is little fear of damage by pressure application, where the resistance value is comparatively low and which is also cheap.
Specifically, the present invention is characterized in that it is a carbon fibre paper used in a polymer electrolyte fuel cell in a state in which short carbon fibres are bound with a polymer material and taking the thickness as X mm and the thickness when 2.9 MPa pressure is applied as Y mm, at least 95% of the short carbon fibres excluding those of length (Y+0.1) mm or less satisfy the relation Wxe2x89xa75X.
Taking the length of the short carbon fibre as W mm and the average length of the short carbon fibre excluding fibre of length (Y+0.1) mm or less as Z mm, the aforesaid carbon fibre paper preferably satisfies the relation Zxe2x89xa75X.
Furthermore, it is preferred that the short carbon fibre be substantially randomly oriented within a two dimensional plane.
Moreover, preferably the relation between the short carbon fibre diameter D (xcexcm), the tensile strength a (MPa) and the tensile modulus E (MPa) satisfies the following relation.
"sgr"/(Exc3x97D)xe2x89xa70.5xc3x9710xe2x88x923
Additionally, it is preferred that the average length of the short carbon fibre be at least 4.5 mm and at least seven times the thickness of the carbon fibre paper, and that the following relation be satisfied.
"sgr"/(Exc3x97D)xe2x89xa71.1xc3x9710xe2x88x923
Furthermore, it is preferred that the short carbon fibre be short fibre of polyacrylonitrile-based carbon fibre, and it is preferred that the diameter of this short carbon fibre be no more than 20 xcexcm and that the volume resistivity in the short carbon fibre lengthwise direction be no more than 200 xcexcxcexa9.m.
Again it is preferred that the reduction in weight be no more than 3% when a uniform pressure of 2.9 MPa is applied in the thickness direction for 2 minutes and then the pressure removed.
Moreover, it is preferred that the resistance be no more than 50 xcexcxcexa9.cm2 when a uniform pressure of 2.9 MPa is applied.
It is also preferred that the thickness is 0.02 to 2.0 mm and that the density lies in the range 0.3 to 0.8 g/cm3 when a uniform pressure of 2.9 MPa is applied in the thickness direction, and it is preferred that the weight per unit area be in the range 10 to 100 g/m2.
Additionally, it is preferred that the polymer material content lies within the range 2-30 wt % and it is preferred that fine carbon particles also be included.
With regard to the current collector employing the carbon fibre paper of the present invention, individual units are constructed by arranging the current collector and catalyst layer in the form of layers, and the polymer electrolyte fuel cell is constructed from a stack containing a plurality of such units, and a moving body such as a motor vehicle or the like may be driven by means of this polymer electrolyte fuel cell.
A current collector employing the carbon fibre paper of the present invention is produced, for example, by a method containing a stage in which, prior to the formation of the catalyst layer on the current collector, the carbon fibre paper is simultaneously heated and pressure applied perpendicular to the plane of the carbon fibre paper, and preferably, prior to the formation of the catalyst layer on the current collector, the carbon fibre paper is brought into contact with a liquid and the pressure applied in a state with the carbon fibre paper soaked with liquid.